There's a theory that all the noise that we're pumping into the seas — sonar, drilling, and more — may be disorienting for sea mammals like whales that navigate by sound. But there's something else that humans already did to the ocean, according to a new study. We killed off almost all the mammals that were there to begin with, who made the oceans a pretty raucous place.

This is the theory of researchers Michael Stocker and Tom Reuterdahl, of Ocean Conservation Research, who are presenting their evidence at the 164th of the Acoustical Society of America.

In 1975, scientist Donald Ross noted that there had been an increase in human-created ambient noise in the ocean by 0.55dB/year between 1958 and 1975 — and it's now as much as 12dB higher than it was in the 50s. Some scientists believe that this noise might be interfering with sea mammals that rely on high frequency sound for navigation and communication.

That's where Stocker and Reuterdahl come in. They think that 200 years ago, the ocean may have been louder than it is today. But we never heard it. Fishing and hunting meant many of the mammals were killed — as much as 90% of the vertebrate biomass in the oceans. In the early 1800s, all of those animals would have been making a considerable amount of noise.

"In one example, 350,000 fin whales in the North Atlantic may have contributed 126 decibels –- about as loud as a rock concert – to the ocean ambient sound level in the early 19th century," said Stocker in a press release.

That's not to say that human-created noises and those of aquatic mammals would be the same. We could still be disorienting whales with our bizarre and intrusive industrial sounds. But it's incredible to think that the oceans may once have been filled with a cacophonous background chatter of animal noise.